I mentioned this suggestion a while ago on a thread in the map making forum, but thought I'd give it it's own wish list topic.
There is room for argument that fliers are already too powerful (most powerful units in game, able to use roads??, no docking cost, mobility, mobility, mobility) and a map maker has very little control over how fliers are to be handled short of banning them altogether. (oh the terrain looks tough, I'll just buy fliers and why use boats?)
The move cost of ridges is currently 4, the same as mountains and like mountains provides a convenient perch for fliers to avoid attack. Where mountains are at least 3 tiles wide (and a penalty of at least 6 moves to cross), they create an obstacle for fliers to slow them down or fly around, ridges are always just 1 tile wide and therefor provide only a minimal inconvenience of 2 extra move points in a flight path, there are steeper costs for land units across regular terrains . The benefits (perching) on the ridge for fliers currently outweighs the penalty cost for using it.
If the move cost of the ridge terrain type was increased to 8 or at least 6, I believe, would improve map play-ability and give more options in map design. I don't believe it should be impassable (although an additional wall tile-set would be nice and lava has it's uses) but fortifications or obstacles designed by a map author should have at least some significant effect on fliers without having to resort to lava.
I don't believe 8 is extreme if you look at the units individually and keep in mind that the perching benefit remains (option of park then attack with full moves), Crows and Ghosts it would merely be a speed bump and leaves plenty move left to do scouting or ambush duty on the other side. Eagles and pegasi's would be hampered but would still have room to launch an attack up to 10 moves away. Where the 8 cost would take it's toll would be the Griffons, Archons, and Demons as their moves would be at least halved but that is the point, they're the heavy hitters and should be slowed down, but they will still have about as many moves as a pikeman or a dwarf left over and It's basically not even an obstacle for the dragons 22 and 26 moves.
I believe this improvement would increase the complexity and play-ability of maps like "The Waste", "Highland Vale", "The Pass" and parts of maps like "Cresendo" and "Dilf G.". I also believe that if this change was implemented it would see more use in other maps as well, fortifying certain cities or dividing starting areas.